E ffi cient and stable CH

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Journal of Power Sources

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E ffi cient and stable CH

₃

NH

₃

PbI

_3-x

(SCN)

_x

planar perovskite solar cells fabricated in ambient air with low-temperature process

Zongbao Zhang

^a

, Yang Zhou

^a

, Yangyang Cai

^a

, Hui Liu

^a

, Qiqi Qin

^a

, Xubing Lu

^a

, Xingsen Gao

^a

, Lingling Shui

^a

, Sujuan Wu

^a,∗

, Jun-Ming Liu

^b

aInstitute for Advanced Materials, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

bLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

H I G H L I G H T S

•

Planar perovskite solar cells (PSCs) have been fabricated in ambient air.

•

NaSCN and KSCN are used as additives in PSCs based on CH₃NH₃PbI_3-x(SCN)_x.

•

Improved micrograph and enhanced charge transfer after incorporating NaSCN/KSCN.

•

Reduced charge recombination, improved efficiency and stability of NaSCN/KSCN-PSCs.

A R T I C L E I N F O

Keywords:

Air-processed CH3NH3PbI3-x(SCN)x

Low temperature Stability

Alkali salts additives Planar perovskite solar cells

A B S T R A C T

Planar perovskite solar cells (PSCs) based on CH₃NH₃PbI_3-x(SCN)_x(SCN: thiocyanate) active layer and low- temperature processed TiO2films are fabricated by a sequential two-step method in ambient air. Here, alkali thiocyanates (NaSCN, KSCN) are added into Pb(SCN)₂precursor to improve the microstructure of CH₃NH₃PbI_3-

x(SCN)xperovskite layers and performance of the as-prepared PSCs. At the optimum concentrations of alkali thiocyanates as additives, the as-prepared NaSCN-modified and KSCN-modified PSCs demonstrate the effi- ciencies of 16.59% and 15.63% respectively, being much higher than 12.73% of the reference PSCs without additives. This improvement is primarily ascribed to the enhanced electron transport, reduced recombination rates and much improved microstructures with large grain size and low defect density at grain boundaries.

Importantly, it is revealed that the modified PSCs at the optimized concentrations of alkali thiocyanates ad- ditives exhibit remarkably improved stability than the reference PSCs against humid circumstance, and a con- tinuous exposure to humid air without encapsulation over 45 days only records about 5% degradation of the efficiency. Thesefindings provide a facile approach to fabricate efficient and stable PSCs by low processing temperature in ambient air, both of which are highly preferred for future practical applications of PSCs.

1. Introduction

Organic-inorganic hybrid methylammonium (CH3NH3PbX3; X = Cl, Br, I) perovskite materials have been the focus of emergingfields such as next generation of photovoltaics[1–4], light emitting diodes[5–8], and photo-detectors [9–12] due to the high absorption coefficients (∼10⁵cm⁻¹) [5,6], wide absorption range[1,2], long electron/hole diffusion lengths (100–1000 nm) [9,12], and appropriate band gap [2,4,13]. The solar cells based on perovskite materials have demon- strated remarkable progress and the efficiencies have achieved from 3.8% to 22.1% in only several years[14–17]. In spite of the excellent

photovoltaic performance, the instability of perovskite CH3NH3PbX3

(X = Cl, Br, I) to water and ambient moisture remains the critical challenge for high efficiency perovskite solar cells (PSCs), in prior to commercialization [18]. Great efforts have been made to avoid the perovskite material decomposition in moist atmosphere, including avoiding the ingression of moisture water into the PSCs by utilizing polymer layers such as PMMA[19,20], and PEG[21], and introducing hydrophobic fullerene derivatives as electron transport materials be- tween perovskite and photocathode [22,23]. However, the essential vulnerability of the perovskite materials to water remains unresolved.

Thiocyanic (SCN) is a kind of pseudohalide, which has the similar

https://doi.org/10.1016/j.jpowsour.2017.11.070

Received 27 September 2017; Received in revised form 1 November 2017; Accepted 23 November 2017

∗Corresponding author.

E-mail address:sujwu@scnu.edu.cn(S. Wu).

0378-7753/ © 2017 Elsevier B.V. All rights reserved.

T

ionic radius to I⁻and the chemical behaviors and properties close to halide. Moreover, interaction between Pb²⁺ and SCN⁻ is much stronger than that between Pb²⁺and I⁻, which offers the PSCs based on CH3NH3PbI3-x(SCN)x active layer benign resistance to moisture [24–27]. Along this line, stable PSCs based on CH3NH3PbI3-x(SCN)x

active layer have been successfully prepared [25,28]. However, the highest power conversion efficiency (PCE) in these PSCs is only 8.3%

and 11.07%, respectively. Very recently, Tai et al. have fabricated a type of efficient and stable PSCs, yielding the average efficiency of 13.49 ± 1.01% in optimum conditions. However, the CH3NH3PbI3- x(SCN)xlayer was deposited on high-temperature annealed mesoporous TiO2films[26]. Certainly, electron transport layer (ETL) prepared by simple and low-temperature process will benefit a lot to reduce the cost of PSCs, which is also highly favorable for flexible devices. Meso- structured ETL usually needs high-temperature annealing (> 450 °C) that hampers the feasibility onflexible plastic substrates, while planar PSCs can be easily prepared by low-temperature process.

On the other hand, one of the main issues in the device fabrication is the control of morphology and crystallinity of the perovskite layer.

Incorporating additives into the perovskite solution has been widely used because of its feasibility and facility, in order to improve the mi- crostructure of the PSCs [29]. Several additives including 1,8-diio- dooctane (DIO)[30,31], ammonium chloride (NH4Cl)[32], hydroiodic acid (HI)[33,34], hydrochloric acid (HCl)[34], methylammonium io- dide (MAI)[35,36], potassium chloride (KCl)[37]and caesium iodide (CsI) [38], sodium iodide (NaI)/sodium bromide (NaBr) [29], have been used to develop smooth, homogenous and continuous perovskite films so that the photovoltaic performance of the as-prepared PSCs can be enhanced.

It has recently been reported that additives of alkali metal halides could benefit to improve the morphology and crystallinity of CH3NH3PbI3films, thus promoting charge generation and dissociation [29,37]. Along this line, for PSCs based on the CH3NH3PbI3-x(SCN)x

active layer, it is natural to choose alkali thiocyanates (NaSCN and KSCN) instead of alkali metal halides themselves as the additives for functionalizing the similar effects as described above. Although the effects of alkali metal halides on the CH3NH3PbI3films for the inverted PSCs fabricated in glove box circumstance were investigated earlier; so far, there has been no detailed results on the role of NaSCN and KSCN additives in obtaining stable and high performance planar PSCs based on CH3NH3PbI3-x(SCN)x active layer and low-temperature processed TiO2ETL in ambient air.

Based on these considerations, we intend to propose a method to fabricate efficient and stable planar PSCs based on air-processed CH3NH3PbI3-x(SCN)xperovskite layer and low-temperature processed TiO2ETL. Our motivation is based on two aspects of consideration. On one hand, the NaSCN and KSCN are chosen as additives into the Pb (SCN)2precursor solution to improve the morphology, crystallinity and stability of the CH3NH3PbI3-x(SCN)xfilms. On the other hand, the low- temperature processed TiO2 ETL allows the whole planar PSCs with high stability and performance to be fabricated on proper flexible substrates, offering particular advantages for easy and cost-competitive commercialization. Our PSCs consist of FTO/TiO2/CH3NH3PbI3- x(SCN)x/Spiro-OMeTAD/Au structure. For convenience of presentation, these structures are abbreviated as“the referencefilms/PSCs”in which no any alkali thiocyanate additive is included,“NaSCN-films/PSCs”and

“KSCN-films/PSCs” in which the NaSCN and KSCN additives are re- spectively added into the Pb(SCN)2precursor solution to fabricate the PSCs. It is shown that the as-prepared NaSCN-films and KSCN-films do have large grain size and homogenous morphology, resulting in en- hanced photovoltaic performance and stability in humid air circum- stance. While the PCE of the reference PSCs is 12.73%, the NaSCN-PSCs and KSCN-PSCs at the optimized concentrations of 2.5% NaSCN and 3.5% KSCN, respectively, show the PCE of 16.59% and 15.62%. More importantly, the NaSCN/KSCN-PSCs show a superior long-term stabi- lity. Upon an exposure to humid air for 45 days without encapsulation,

the PCE of the KSCN-PSCs and NaSCN-PSCs can respectively retain 97%

and 93% of the initial values, but the PCE of the reference PSCs can maintain 87%. The PCE enhancement in the KSCN/NaSCN-PSCs is mainly attributed to the remarkably improved microstructure and crystallinity of the CH3NH3PbI3-x(SCN)x layers. Furthermore, the highest processing temperature is only 200 °C.

2. Experimental section 2.1. Devices fabrication

Material synthesis, device fabrication, and characterizations were all carried out in ambient air. CH3NH3I (MAI) was synthesized as re- ported previously[39]. FTO glass (15Ω/square, NSG) was etched with Znic powder and diluted hydrochloric acid, then cleaned successively with detergent, deionized water, acetone and iso-propanol by ultra- sonication for 20 min. The TiO2compact layer was deposited on the FTO substrate [40]. Fig. 1a shows the fabrication process of CH3NH3PbI3-x(SCN)x layer. First, Pb(SCN)2 powder (Sigma-Aldrich) was dissolved in dimethylsulfoxide (DMSO, Sigma-Aldrich) at 500 mg/

ml and stirred overnight at temperature of 60 °C on a hot plate, then filtered with 0.22μm nylonfilter to obtain a clear solution. Then the solution was spun on the FTO/TiO2substrate at a spin-rate of 4000 rpm for 30 s in humid air. The Pb(SCN)2layer was annealed at 90 °C for 30 min in ambient air. Then MAI (8 mg/ml dissolved in anhydrous isopropanol) was dropped on the top of dried Pb(SCN)2layer and kept for 20 s, followed by spin-coating at 3000 rpm for 60 s. The stacked precursor layers of Pb(SCN)2and CH3NH3I were then annealed on a hot plate at 80 °C for 20 min in atmosphere.

For the NaSCN or KSCN additive, different amounts of NaSCN (Aladdin) or KSCN (Aladdin) (dissolved in DMSO) were added to the Pb (SCN)2precursors and thefilms were fabricated in the same method as described above. After the annealing, a thin layer of 2, 2′, 7, 7′- Tetrakis (N,N-di-p-methoxyphenylamine) - 9, 9′-spirobifluorene (Spiro- OMeTAD) was deposited on the CH3NH3PbI3-x(SCN)xlayer by spin- Fig. 1.(a) A schematic drawing of fabrication of CH3NH3PbI3-x(SCN)xfilms, (b) XRD patterns of the referencefilm, NaSCN-film and KSCN-film deposited on FTO/TiO2, re- spectively.

coating a chlorobenzene solution containing 80 mM Spiro-OMeTAD, 64 mM tert-butylpyridine (TBP), and 24 mM Li-bis(tri- fluoromethanesulfonyl)-imide (Li-TFSI) (520 mg/ml in acetonitrile) at 4000 rpm for 30 s. Ultimately, an Au electrode was evaporated on each sample surface through a shadow mask under a vacuum of 10⁻⁴Pa. To this end, the PSCs were fabricated. The sample size was 0.045 cm². 2.2. Characterizations

The photovoltaic performance of these PSCs was characterized using a Keithley 2420 source meter under an illumination of 100 mW/

cm²(Newport 91160 equipped with an AM 1.5 Gfilter). The light in- tensity was calibrated by a standard silicon solar cell (certified by NREL). The crystallinity and morphology of the perovskitefilms were studied using the X-ray diffraction (XRD) (X'Pert PRO, CuKαradiation) and scanning electron microscopy (SEM, ZEISS ULTRA 55). The UV-vis absorption spectra were measured by using a SHIMADZU UV-2550 spectrophotometer. The external quantum efficiency (EQE) curves were measured using a standard EQE system (Newport 66902). The photo- luminescence (PL) spectra were measured by a fluorescence spectro- photometer (HITACHI F-5000) exited at 470 nm. And the PL spectra were normalized to the absorbance and measured in the same condi- tions. The electrochemical impedance spectroscopy (EIS) and intensity- modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements were performed on the Zahner Zennium electrochemical workstation. For the EIS measurements, a 20 mV ac-sinusoidal signal source was employed over the constant bias with the frequency ranging from 0.5 Hz to 1.0 MHz under 100 mW/cm²white light. For the IMPS and IMVS characterization, a red LED light (λ= 632.2 nm) was used to provide the sinusoidal optical perturbation signal, whose amplitude was 10% of the background light intensity with the frequency ranging from 0.5 Hz to 1.0 MHz.

3. Results and discussion

We investigate the effect of NaSCN and KSCN additives on the morphology and crystallinity of the NaSCN- and KSCN-films deposited on the TiO2ETL, prepared by a sequential two-step method in ambient air. The XRDθ-2θpatterns for the referencefilm, the NaSCN-film with 2.5% NaSCN and KSCN-film with 3.5% KSCN, respectively, are shown inFig. 1b. As illustrated inFig. 1b, the peaks at 2θ= 14.2°, 28.5° and 31.8°, corresponding to the (110), (220) and (310) reflections, respec- tively, demonstrate the formation of tetrahedral perovskite crystal structure[3,4,28,41]. An introduction of SCN⁻shows similar effect as the incorporation of Cl does, implying no remarkable variation in terms of the perovskite structure[28,42]. The XRD patterns for the NaSCN- film and KSCN-film do not show significant shifting in the peak posi- tions. Moreover, the main peaks in the NaSCN-film and KSCN-film show considerably stronger intensity than that of the reference film, con- firming the contribution of NaSCN and KSCN additives to the perovskite crystallization[28]. The peak at 12.55° corresponds to the character- istic peak of PbI2, which is the product during the formation of per- ovskitefilm[41]. It was reported that amount of PbI2could compress the charge recombination and enhance the performance of PSCs [42,43].

The obtained positive effect stimulates us tofind out the optimum concentrations of NaSCN and KSCN additives to the Pb(SCN)2precursor solutions, in terms of the performance of the PSCs with the identical TiO2 ETL. Fig. S1 and Fig. S2 (Supporting Information) show the measured photocurrent density-voltage (J-V) curves and PCE as func- tions of the additive concentration, respectively. It can be seen that the NaSCN-PSCs with 2.5% additive and KSCN-PSCs with 3.5% additive show the highest efficiency. Hereafter, we fix the NaSCN the con- centration at 2.5% and the KSCN concentration at 3.5%, and the measured results are shown inFigs. 1–6andFigs. S1–S7.

First, the morphology of the as-prepared CH3NH3PbI3-x(SCN)xfilms

was investigated. The top-view and cross-sectional SEM images of the films deposited on TiO2ETL are presented inFig. 2.Fig. S3(Supporting Information) shows the particle size distribution histograms obtained fromFig. 2. The grain size of the referencefilms is small (∼250 nm) with high density grain boundaries, as shown inFig. 2a/d. For the NaSCN/KSCN-film, the grain size is significantly increased and the evaluated values are∼450 nm and 500 nm, respectively, as shown in Fig. 2b/e andFig. 2c/f. It is also consistent with earlier report that the perovskitefilm has larger crystallites and more compact morphology than that of Na⁺additive[29]. For high efficiency PSCs, less defective films with large and regular grains are necessary[29,37].

It is noted that the nonvolatile nature of NaSCN and KSCN salts make the alkali ions in thefilms less in loss[37,44], as confirmed by SEM mapping measurements (see Supporting Information fromFigs.

S4–S6). Moreover, the surface morphology and roughness of thesefilms were checked using atomic force microscopy (AFM) and the images are shown inFig. S7in the supporting information. Remarkable variations in the surface features upon the NaSCN and KSCN additives are iden- tified. The root mean square (RMS) values are 40.02 nm, 38.46 nm, and 29.04 nm for the referencefilm, NaSCN-film and KSCN-film, respec- tively. The reduced RMS value in the latter twofilms is believed to benefit to the interfacial spreading between Spiro-OMeTAD and CH3NH3PbI3-x(SCN)x layers, resulting in the improved photovoltaic performance[45,46].

To investigate the effects of NaSCN and KSCN additives on the performance of the PSCs, with the FTO/TiO2/CH3NH3PbI3-x(SCN)x/ spiro-OMeTAD/Au structure, as illustrated inFig. 3a, a set of measured data are presented here. TheJ-Vcurves for the reference PSC, NaSCN- PSC and KSCN-PSC are plotted inFig. 3b. The corresponding photo- voltaic parameters are summarized inTable 1. The reference PSCs ex- hibit an average PCE of 11.82% and the best PCE is 12.73%, compar- able with that for optimized PSCs prepared by a sequential two-step process under humid atmosphere, using the similar mesoporous archi- tectures[26]. With the optimized KSCN additive (3.5%), the champion PCE of the KSCN-PSCs reaches 16.59% and the average PCE is 15.94%, while the best PCE of the NaSCN-PSCs with the optimized NaSCN ad- ditive (2.5%) is 15.63% and the average PCE is 14.75%, as shown in Figs. S1 and S2of the supporting information.

The other measured PSC parameters including the open voltage (Voc), short-current density (Jsc), and fill factor (FF) are presented in Fig. 3. The efficiency distribution histograms for 50 PSCs are presented inFig. 3c. It is reasonable to attribute the better performance of the NaSCN/KSCN-PSCs to the improved morphology and crystallinity of the CH3NH3PbI3-x(SCN)x films upon the additions of NaSCN and KSCN [28]. In order to understand the consequences induced by the NaSCN and KSCN additives, the measured external quantum efficiency (EQE) and the absorption spectra for the reference PSC and KSCN- and NaSCN- PSC are presented inFig. 3d and e, respectively. The measured EQE and absorption spectra for these PSCs show the higher EQE and stronger light absorption in the range of 350–750 nm for the KSCN- and NaSCN- PSC, with respect to the reference PSC, thus resulting in the enhanced Jsc[47]. The measured FF and the ratio of shunt resistance (Rsh) to series resistance (Rs), Rsh/Rs, for these PSCs have the one-to-one cor- respondence. The higher FF in the NaSCN-PSC and KSCN-PSC is cer- tainly due to the larger Rsh/Rs[48].

For better understanding of the electron transport and recombina- tion in these PSCs, the electrical impedance spectroscopy (EIS), pho- toluminescence spectra (PL), and intensity-modulated photocurrent and photovoltage spectroscopies (IMPS/IMVS) are carried out. The Nyquist curves measured under illumination are given inFig. 4a where the solid lines are thefitting results using the model inserted. For a reliablefit- ting, the constant phase angle element (CPE) instead of ideal capaci- tance C is used to take account of the spatial inhomogeneities induced by defects and impurities on the interface[36]. The significant differ- ence between the reference PSCs and KSCN/NaSCN-PSCs is shown. The size of the semicircles in the KSCN/NaSCN-PSCs is bigger than that of

the reference PSCs. Since the semicircle is related to the charge re- combination in the TiO2/CH3NH3PbI3-x(SCN)x/spiro-OMeTAD inter- faces[49], one may obtain the recombination resistance (Rrec) for these PSCs by the modelfitting inserted inFig. 4a. The KSCN/NaSCN-PSCs exhibit larger Rrecthan that of the reference PSCs, being favorable for the device performance[26,36]. The PL spectra can be used to explore the trap states and recombination properties of photo-excited charges in semiconductors [50–52]. The measured PL spectra for our reference

and KSCN/NaSCN-films are plotted inFig. 4c. The reference film ex- hibits the highest intensity, suggesting the highest charge recombina- tion rates, which are obviously lower in the KSCN/NaSCN-films [36,51,52], consistent with the results of the EIS. These data confirm that adding KSCN and NaSCN into the Pb(SCN)2precursor does reduce the charge recombination rates in the perovskitefilms, contributing to the enhanced charge collection and thus improved performance[26].

On the other hand, the measured IMPS and IMVS data are plotted in Fig. 2.Top and cross-sectional SEM images of the CH3NH3PbI3-x(SCN)xfilms deposited on TiO2/FTO/glass: (a) and (d) referencefilm, (b) and (e) KSCN-film, (c) and (f) NaSCN-film. The scale bars are 400 nm.

Fig. 3.(a) A schematic draw of the PSC structure, (b) The measured J-V curves, (c) Efficiency statistics histograms (50 PSCs for each batch), (d) EQE spectra (e) Absorption spectra (f) Rsh/Rsand FF of the reference PSC, KSCN-PSC and NaSCN-PSC, respectively.

Fig. 5a and b. The electron transport time constantτtis calculated by equationτt= 1/2πft, whereftis the minimum characteristic frequency obtained from the IMPS curves[53,54]. As shown inFig. 5a, the two distinct semicircles appear over the whole frequency range, suggesting two different transport processes. As the earlier reports[55,56], the

semicircle at low frequency (below 10³Hz) is directly connected with the electron transport in TiO2, while the semicircle at high frequency (10⁴–10⁵Hz) is attributed to the charge transport in the CH3NH3PbI3- x(SCN)xfilms[56,57]. The electron recombination time constantτnis calculated from equationτn= 1/2πfn, wherefnis the minimum char- acteristic frequency of the IMVS curves (Fig. 5b)[53,55]. The evaluated τtandτnfor the reference PSCs and the KSCN/NaSCN-PSCs are plotted inFig. 5c. Clearly, the KSCN/NaSCN-PSCs have shorterτtand longerτn

than those of the reference PSC, suggesting the enhanced electron transport and reduced recombination rates in the interface between the TiO2and CH3NH3PbI3-x(SCN)x, due to the reduced grain boundaries in the CH3NH3PbI3-x(SCN)xfilms and their improved coverage on the TiO2

ETL surfaces[54].

The electron diffusion coefficient (Dn) and electron diffusion length (L) for these PSCs were also evaluated, as shown inFig. 5d. The coef- ficientDnis calculated from equationDn=d²/(2.35τt), wheredis the thickness of photoanodefilm[55,58], i.e. the thickness of TiO2layer here. As shown inFig. S8of the supporting information, the thickness of compact TiO2layer (d) is∼65 nm. TheDnvalues in the KSCN/NaSCN- PSCs are larger, indicating the improved electron diffusion[54]. The L value represents the average distance by which an electron can travel before it recombines with either the oxidized perovskite layer or the hole transport material in the cells[58–60]. It depends on bothDnand τnas shown in equationL= (Dn·τn)^1/2. A largerLmeans that a thicker photoanodefilm can be used to promote the PCSs[55]. The values ofL in these PSCs are all larger than the thickness of the TiO2layers, sug- gesting an efficient collection of photo-generated electrons[54,61]. The KSCN/NaSCN-PSCs have the largerL, ascribed to the promoted electron transport and decreased electron recombination rates[53,54,62]. The KSCN-PSCs have the largestDn,τn, andLas well as the smallestτt, and thus the best performance.

Moreover, the highly concerned issue, i.e. the stability of the PSCs against humid circumstance was also studied. In our experiments, all the PSCs were exposed in the circumstance with an average relative humidity over 70%. As shown inFig. 6a, the PCE values of the NaSCN/

KSCN-PSCs after exposed for 45 days in such circumstance remain to be about 93% and 97% of the initial values, respectively. However, the reference PSCs can only retain 87% of the initial PCE. These results indicate that the KSCN/NaSCNePSCs do have highly appreciated sta- bility against the humidity deterioration [26,37]. To understand the stability comprehensively, the samples after the long term exposure were checked again by XRD patterns and SEM images.Fig. 6b shows the variations of XRD spectra at several times. It can be seen that the (110) Fig. 4.(a) The Nyquist plots of the reference PSC, KSCN-PSC and NaSCN-PSC measured

under light illumination. The open symbols represent the experimental data and the solid lines are thefitting results. Insert: the equivalent circuit diagram which is used tofit the data of Nyquist plots. (b) Recombination resistance obtained from thefitting results in Nyquist plots. (c) PL spectra of the referencefilm, KSCN-film and NaSCN-film.

Fig. 5.The measured IMPS curves (a), IMVS curves (b), evaluatedτtandτn(c), evaluated Dn

and L (d) for the reference PSC, KSCN-PSC and NaSCN-PSC, respectively.

and (220) peak intensities of perovskite layer in the reference, KSCN/

NaSCN-films do decay gradually with time, suggesting the degradation of the perovskite crystallinity [63,64]. Notably, the reference films exhibit an increased (001) peak intensity, corresponding to the char- acteristic peak of PbI2[4].Fig. 6c demonstrates the relative intensity of (110) peak for perovskite layer to (001) peak for PbI2(110/001). It can be seen that the ratio decreases with time for all of the PSCs, but this decrease for the KSCN/NaSCN-PSCs is much slower than that for the reference PSC, indicating the dramatic decomposition of the reference perovskitefilms upon the exposure. Moreover, the degradation is neg- ligible over the whole period of exposure up to 30 days in the KSCN/

NaSCN-PSCs. This is consistent with the results in Fig. 6a. The SEM images of the reference and KSCN/NaSCN-PSCs taken from the fresh and 30-day exposure samples, are presented inFig. 6d–i. It is noted that many pinholes are observed in the referencefilm after 30-day exposure, but the pinholes in the KSCN/NaSCN-PSCs are less. Moreover, the grain size in these films becomes smaller over time, due to the possible

decomposition of the perovskite structure. This can explain the stability difference of the reference PSC and NaSCN/KSCN-PSCs.

4. Conclusions

In summary, the planar PSCs based on CH3NH3PbI3-x(SCN)xper- ovskite layer and TiO2electron transport layer have been fabricated by low-temperature process in ambient air. The uniform and compact perovskitefilms with large grain size have been obtained by adding appropriate concentrations of KSCN or NaSCN into the Pb(SCN)2pre- cursor. The champion efficiencies of 16.59% and 15.63% have been obtained for the KSCN-PSC and NaSCN-PSC respectively, higher than the efficiency of 12.73% for the reference PSC. Our results indicate that the improved performance upon the KSCN or NaSCN additives is at- tributed to the large grain size, enhanced crystallinity and optical ab- sorption, increased charge transport and decreased charge recombina- tion rates. Importantly, the KSCN/NaSCN-PSCs demonstrate the significantly improved long-term stability. After being exposed to humid circumstance with 70% relative humidity for 45 days without encapsulation, the efficiencies of the KSCN-PSC and NaSCN-PSC remain to be ∼97% and ∼93% of the initial values, respectively. The en- hanced stability can be ascribed to the slow decomposition of the KSCN/NaSCN-films. This work establishes favorable strategies for fab- ricating the PSCs with high efficiency and stability in humid atmo- sphere by low-temperature process.

Acknowledgements

We acknowledge the financial support of the Natural Science Fig. 6.Evolution of the (a) PCE, (b) XRD patterns, (c) relative intensity of (110) peak for perovskitefilm to (001) peak of PbI2(110/001), SEM images of 0 day (d, e, f) and 30 days exposure (g,h, i) for a reference PSC, a KSCN-PSC and a NaSCN-PSC without encapsulation stored in ambient air. All scale bars are 400 nm.

Table 1

Photovoltaic parameters of reference PSC, KSCN-PSC and NaSCN-PSC.

Alkali Salt Voc(V) Jsc(mA/cm²)^c Jsc(mA/cm²)^d FF(%) PCE(%)

Reference 0.965 19.33 18.93 68.22 12.73^a(11.82)^b

KSCN 1.065 20.45 19.91 76.20 16.59^a(15.94)^b

NaSCN 1.030 20.90 20.46 72.62 15.63^a(14.75)^b

aBest PCE.

bAverage PCE from 50 devices.

cMeasured Jscvalues from the solar simulator.

dIntegrated Jscvalues from the EQE curves.

Foundation of Guangdong Province (No. 2016A030313421), the Characteristic Innovation Project of Guangdong Provincial Department of Education (Science 2016), the National Natural Science Foundation of China (Grant Nos. 51431006, 51472093, 61574065, 21403089), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Science and Technology Planning Project of Guangdong Province (Grant No. 2016B090907001, 2016B090906004, 2015B090927006), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R70), the National Key R & D Program of China (2016YFA0201002), the Guangdong Innovative Research Team Program (No. 2011D039) and the MOE International Laboratory for Optical Information Technologies.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttp://dx.

doi.org/10.1016/j.jpowsour.2017.11.070.

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